Computer Networks – Basic theory of operation

Computers communicate by using the principle of packet switching and next hop routing.Packet switching is the process of splitting data into smaller units called as packets and then giving it to the underlying data exchange (computer network) for transmission. Each packet is an independent entity, having a header and some data. The data exchange devices (Routers) use the header information and next hop routing to switch the packet from the source computer to the destination computer. Next hop routing is the process of choosing a best next hop neighboring Router to pass on a packet to, based on the packets intended destination and the Router’s routing table.

The diagram given below illustrates the basic theory of operation of computer networks:

Each communicating computer has at least one unique logical identifier in the form of an IP address

At the source computer, application data (file, email, web request etc.) in digitized form is split into smaller units called segments

A header containing destination and source IP addresses are then added to each segment to form packets

Each packet is then independently handed over to the underlying computer network to be carried from the source computer to the destination computer

Underlying computer network routes each packet from the source computer to the destination computer using the destination IP address in the packet (packet switching)

Underlying computer network comprises of

a complete end to end communication path between source and destination nodes through intermediate nodes (telecommunication wired/wireless links)

At each node, a routing table is used to choose an appropriate next hop based on packet’s destination IP address

The routing tables are usually built dynamically by exchange of information between routers (routing protocols)

Each node’s routing table picks that neighbor which it thinks is more closer to the actual packet’s destination than itself (next hop routing)

Each node in the path passes on a received packet to one of its directly connected neighbors

The packet is thus passed from node to node till it reaches the destination node

Thus packet switching and next hop routing are used to transfer each packet between source and destination nodes

The digital packet contents are actually passed between nodes by transmission of voltages /currents/light either through wired or wireless media

Also, as illustrated in the diagram, different packets between the same source and destination nodes may not necessarily take the same route, due to the dynamic nature of routing tables and also due to load balancing considerations.